This invention relates generally to rotary machines and more particularly, to methods and apparatus for operating gas turbine engines.
At least some known gas turbine engines combust a fuel and air mixture to release heat energy from the mixture to form a high temperature combustion gas stream that is channeled to a turbine via a hot gas path. The turbine converts thermal energy from the combustion gas stream to mechanical energy that rotates a turbine shaft. The output of the turbine may be used to power a machine, for example, an electric generator or a pump.
At least one by-product of the combustion reaction may be subject to regulatory limitations. For example, within thermally-driven reactions, nitrogen oxide (NOx) may be formed by a reaction between nitrogen and oxygen in the air initiated by the high temperatures within the gas turbine engine. Generally, engine efficiency increases as the combustion gas stream temperature entering a turbine section of the engine increases. However, increasing the combustion gas temperature may facilitate an increased formation of NOx.
Combustion normally occurs at or near an upstream region of a combustor that is normally referred to as the reaction zone or the primary zone. Mixing and combusting of fuel and air may also occur downstream of the reaction zone in a region often referred to as a dilution zone. Inert diluents may be introduced directly into the dilution zone to dilute the fuel and air mixture to facilitate achieving a predetermined mixture and/or temperature of the gas stream entering the turbine section. However, inert diluents are not always available, may adversely affect an engine heat rate, and may increase capital and operating costs. Steam may be introduced as a diluent, however, steam may shorten a life expectancy of the hot gas path components.
To facilitate controlling NOx emissions during turbine engine operation, at least some known gas turbine engines use combustors that operate with a lean fuel/air ratio and/or wherein the combustors are operated such that fuel is premixed with air prior to being admitted into the combustor's reaction zone. Premixing may facilitate reducing combustion temperatures and subsequently reduce NOx formation without requiring diluent addition. However, if the fuel used is a process gas or a synthetic gas, or syngas, the process gas and/or syngas selected may include sufficient hydrogen such that an associated high flame speed may facilitate autoignition, flashback, and/or flame holding within a mixing apparatus. Moreover, such high flame speed may not facilitate uniform fuel and air mixing prior to combustion.